Ledger, who died of an accidental drug overdose in his Manhattan apartment in January, was 17 and a schoolboy in Perth, Western Australia, when he struck up a relationship with a 25-year-old woman, reports said.

The woman allegedly found out she was pregnant with a baby girl after their relationship had ended. By that time she was living with another man.

"There is a very real possibility that Heath was the father," the actor's uncle, Haydn Ledger, told Sydney's Daily Telegraph newspaper.

Haydn Ledger is estranged from his brother Kim, Heath's father, and has defied requests by the family not to speak publicly about the actor's death.

Another member of the family told the paper that any suspicions the child had been fathered by the Brokeback Mountain star were suppressed.

"She had the baby. Everyone lived under the assumption that she was the daughter of the mother's boyfriend and that is how she has been brought up,” the family member said.

If the claims are proved to be true, Ledger's multi-million pound estate would have to be divided between the child he fathered in his teens and his two-year-old daughter, Matilda Rose, whose mother is Hollywood actress Michelle Williams.

At one time he was engaged to Williams, who played his wife in Brokeback Mountain, but the couple later split up.

The actor's parents, Kim and Sally Ledger, have declined to comment on the reports.